Modern gas or combustion turbines must satisfy the highest demands with respect to reliability, weight, power, economy, and operating service life. In the development of such turbines, the material selection, the search for new suitable materials, as well as the search for new production methods, among other things, play an important role in meeting standards and satisfying the demand.
The materials used for gas turbines may include titanium alloys, nickel alloys (also called super alloys) and high strength steels. For aircraft engines, titanium alloys are generally used for compressor parts, nickel alloys are suitable for the hot parts of the aircraft engine, and the high strength steels are used, for example, for compressor housings and turbine housings. The highly loaded or stressed gas turbine components, such as components for a compressor for example, are typically forged parts. Components for a turbine, on the other hand, are typically embodied as investment cast parts.
Although investment casting is not a new process, the investment casting market continues to grow as the demand for more intricate and complicated parts increase. Because of the great demand for high quality, precision castings, there continuously remains a need to develop new ways to make investment castings more quickly, efficiently, cheaply and of higher quality.
Conventional crucibles are not suitable for casting reactive alloys, such as titanium alloys. One reason is because there is a reaction between molten titanium and the crucible. Any reaction between the molten alloy and the crucible will deteriorate the properties of the final casting. The deterioration can be as simple as poor surface finish due to gas bubbles, or in more serious cases, the chemistry, microstructure, and properties of the casting can be compromised.
The challenge has been to produce a crucible that does not react significantly with titanium and titanium aluminide alloys. In this regard, few if any prior poured ceramic investment compounds exist that meet the requirements for structural titanium and titanium aluminide alloys. Therefore, there is a need for a ceramic crucible that does not react significantly with titanium and titanium aluminide alloys. Approaches have been adopted previously with ceramic shell crucibles for melting titanium alloys. In the prior examples, in order to reduce the limitations of the conventional investment crucible compounds, several additional crucible or mold materials have been developed. For example, a mold investment compound was developed of an oxidation-expansion type in which magnesium oxide or zirconia was used as a main component and metallic zirconium was added to the main constituent to compensate for the shrinkage due to solidification of the cast metal. There is thus also a need for simple and reliable melting and investment casting methods which allow easy melting of metals or metallic alloys in an investment crucible that does not react significantly with the metal or metallic alloy.
Induction melting generally involves heating a metal in a crucible made from a non-conductive refractory alloy oxide until the charge of metal within the crucible is melted to liquid form. When melting highly reactive metals such as titanium or titanium alloys, vacuum induction melting using cold wall or graphite crucibles is typically employed in preference to oxide based ceramic crucibles.
Difficulties can arise when melting highly reactive alloys, such as titanium alloys, as a result of the reactivity of the elements in the alloy at the temperatures needed for melting. While most induction melting systems use refractory alloy oxides for crucibles in the induction furnace, alloys such as titanium aluminide (TiAl) are so highly reactive that they can attack the crucible and contaminate the titanium alloy. For example, ceramic crucibles, such as alumina-, magnesia-, and silica-containing crucibles, are typically avoided because the highly reactive alloys can react with the crucible and contaminate the titanium alloy with oxygen. Similarly, if graphite crucibles are employed, both the titanium and titanium aluminide based alloys can dissolve large quantities of carbon from the crucible into the titanium alloy, thereby resulting in contamination. Such contamination results in the loss of mechanical properties of the titanium alloy.
Cold crucible melting offers metallurgical advantages for the processing of the highly reactive alloys described previously, it also has a number of technical and economic limitations including low superheat, yield losses due to skull formation, high power requirements, and a limited melt capacity. These limitations can restrict its commercial viability.
Accordingly, there remains a need for ceramic crucibles for use in melting highly reactive alloys that are less susceptible to contamination and pose fewer technical and economic limitations than current applications.